Syncytin

The organ a virus built is still inside you. Right now, if you are pregnant, or if you were ever carried by someone who was, it kept you alive — and it was not invented by your ancestors. It was stolen from a virus.

Here is what happened. A retrovirus infected a small mammal somewhere in the Cretaceous. The virus did what viruses do: it wrote itself into the animal's DNA and went quiet. Most viral stowaways are silent cargo. This one wasn't. One of its genes — the gene the virus used to fuse its outer coat to a cell wall, the mechanism of infection itself — turned out to be exactly the right tool for a different job. The embryo needed to anchor itself to the mother. It needed cells that could fuse, reach, grip. The viral gene did this perfectly.

The animal survived. The gene passed to her daughters. And her daughters' daughters. For a hundred million years the descendants of that infection have been using a hijacked weapon of invasion to build the placenta — the organ that decides what the mother feeds her child and what she keeps for herself.

Every placental mammal alive — every whale, every elephant, every person — carries this gene. We did not evolve it. We caught it. And without it, none of us exist.

We still don't know how many other things viruses quietly built inside us while we weren't looking.